The present invention relates generally to power converters and, more particularly, to a dc-to-dc converter having overvoltage protection.
To be practicable, off-line switching converters supplied from an ac line need low-cost switching devices with very high breakdown voltage. The availability of such components is limited. Considering, for example, a 480 Vac line voltage with a +20% upper voltage limit, a maximum dc bus voltage value can be as high as 800 Vdc. Assuming a single-switch dc-to-dc converter topology as a most economical solution, such a switching device would need a breakdown voltage of over 1000V.
Presently, low-cost transistors (e.g., insulated gate bipolar transistors (IGBT""s) and MOSFET""s) are available with a breakdown voltage rating of up to 1,600V. Disadvantageously, such switching devices can easily fail if the dc bus voltage exceeds 1,000V during an input voltage transient. For overvoltage pulses of short duration, the dc bus voltage can be limited by metal-oxide varistors that offer effective and economical protection against lightning surges and other fast transients. Such metal-oxide varistors, however, do not have sufficiently high energy ratings to effectively guard against certain longer-lasting input voltage transients.
In North America, an ungrounded wye-delta transformer connection is commonly used to supply 480 Vac to light industrial loads. When a primary phase is disconnected from the source, the transformer can generate severe overvoltages that may persist for as long as the open-phase condition is present. Two distinctly different phenomena can cause such an overvoltage condition: (1) a shift of the neutral voltage due to load imbalance; or (2) ferroresonance due to interaction between the transformer core saturation characteristics and the transformer winding capacitance. It has been documented that an overrvoltage magnitude can be 2.5 times higher than a nominal voltage and can last as long as 30 seconds. In such an environment, a metal-oxide varistor could not be used as a protection device.
Other available methods of overvoltage protection, including (1) various combinations of circuit breakers and surge arresters and (2) protective devices in combination with Zener diodes, disadvantageously require resetting.
Accordingly, it is desirable to provide protection for a dc-to-dc converter from overvoltages while providing normal output voltage. It is furthermore desirable that such protection not require resetting after occurrence of an overvoltage.
A dc-to-dc converter is provided with overvoltage protection circuitry while still providing normal output voltage. Such overvoltage protection circuitry advantageously does not require resetting. In exemplary embodiments, the overvoltage protection circuitry is connected between an input rectifier bridge and a dc output voltage bus, and comprises a first switching device connected between the input bridge and the dc output voltage bus and further comprises an overvoltage current path. The overvoltage current path comprises a complementary switching device in series with Zener diode circuitry having a predetermined breakdown voltage. The overvoltage current path further comprises a transition current path. The first switching device is closed for nominal input voltage and is open for overvoltages. When the input voltage exceeds the Zener diode breakdown voltage, the transition current path conducts current. Then, when the threshold voltage of the complementary switching device is exceeded, the complementary switching device turns on and current flows through the overvoltage protection path formed by complementary switching device 30 and the Zener diode circuitry. After the overvoltage transient condition subsides and the input voltage decreases back to the nominal level, the complementary switching device turns off, and normal converter operation resumes.